1. Field of the Invention
The present invention relates to a method for purifying pure water or ultrapure water. The purified pure water and the ultrapure water are used, for example, as a solvent for etching or washing or for making electrolytic ionic water
2. Description of the Related Art
Electrolytic ionic water made by an electrolytic ionic water production apparatus has been used in various fields and particularly has been extensively used in the manufacture of semiconductor devices and the manufacture of liquid crystal devices.
Conventionally, fluorine-based solvents such as Freon, CCl.sub.3 F, have been used for washing semiconductor substrates in the manufacture of semiconductor devices. However, because these solvents have adverse affects on the environment they are sometimes avoided, and in their place a safer solvent, water, such as water as pure water or ultrapure water is used. Pure water is water of a high purity of about 5 M.OMEGA.cm to 18 M.OMEGA.cm in resistivity. Pure water has almost all impurities such as ions, small particles, microorganisms and organic substances removed from it. Ultrapure water is water of an extremely high purity, higher than that of pure water, from which suspended matter and dissolved substances have been removed by means of an ultrapure water production apparatus. Ultrapure water has a higher resistivity than pure water.
When pure water or ultrapure water is electrolyzed, the resulting product is electrolytic ionic water such as an anode ionic water, which is strongly oxidized (acidic water), and cathode ionic water, which is strong reduced (alkaline water).
Of interest in the manufacture of semiconductor devices and liquid crystal devices and the like is washing and polishing semiconductor substrate surfaces using electrolytic ionic water, such as acidic ionic water or alkaline ionic water produced from pure water or ultrapure water.
Conventionally, pure water and ultrapure water have been highly purified using an ion exchange resin technique. However, with this method, not all elements are completely removed. In particular, positive ions of sodium, potassium, calcium and zinc and the like are not easily removed even with an ion exchange resin technique. When ultrapure water is used for washing a semiconductor wafer, mobile ions such as these are adsorbed onto the wafer and cause capacitance variations and reduction in the reliability of insulating films on the wafer, hindering the manufacture of semiconductor devices.